E=MC^2 Question

Does this mean if I had a rubber band and weighed it, then stretched and weighed it while being stretched out, that it would weigh more at that time? Since at that time it has stored energy, which equates to mass?

If you stretch out a rubber band the energy is converted into elastic potential energy. The fact that the rubber band is stretched out proves that the vast majority of the energy cannot go anywhere else, eg. mass.